Limiter output control



Dec. 16, 1941.

D. E. FOSTER ETAL 'LIMTTER OUTPUT CONTROL Filed Oct. 2, 1940 AAAAAAA AAA N, m ...um d @L w f als Z Patented Dec. 16, 1941 `UNITfl'lel1) STATES PATENT oFF-Ice 4 i Y l LIMITEE ozi'lsrrl CONTROL A. Rankin, Port Washington,N. Y., kassignors to Radio Corporation of'America, a corpora.-

tion of Delaware Application october 2,1940, seriaiNo. 359,330

One of thejmain objects of our inventionis to* providea device' for varying the limiter out.

putfoii, an FM receiver/thereby to secure4 gain control .actionyand at the same time maintaining the limiting action. Y v g lAnother important object of the inventionfis toprovide a -volume control for an FM receiver; the` control Y essentially comprising a device for adjusting ,thev limiter tube `input-output characteristic over -a family gif-characteristics. c

Another object of our invention is to provide ,a manual volume-.control instrumentality for an receiver, wherein the limiter tube screen potential may be varied in such a manner as to produceY -a variation in the effective threshold value of thelimiter characteristic. Y

VStill'other'objects of the invention are to improveA generally the simplicity and efciency of Y gain controls for' `FM receivers, and more especially to provide a highlyy simple type of volume control device whichcis economically manufactured andassembled in an FM receiver.

'Ihe novel l features Awhich we believe to be characteristic of our invention'are set forth in particularityin-the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood-byj reference to the following description taken inconnection with Ythe drawing in which we have indicated diagrammatically a circuit organization whereby our inventionmay be ycarried into effect.y Y

In the drawing: c Y Fig. lk shows an embodiment of the invention, Y

Y Figs. 2 and 3- graphically illustrate the func--k tioning of the invention.

Referring now .to the yaccompanying drawing, thereisshown in detail the limiter stage ofL an FM receiver olf the superheterodyne type. Let it beassumed that the receiver is constructed to receivefsignals in the 43 to 50 megacycle (mc.) band of VlilVI reception; .and that it uses an intermediate frequency (I. F.) of 2.175 mc. The networks preceding the limiter input circuit I may comprise the customary signal collector feeding into one or more'tunable radio frequency amplifiers, vanda tunable local oscillator feeding. local oscillations to Vthe'iirst detector alongWith the amplified VFM signals' fromthe ampliers.

(Cl.Y 25th-20) The FM signals at I. F. value may be vpassed through one or more I. F. ampliers each operating at the 2.175 mc. frequency. The circuit 2, tuned to the I. F. value, may be considered as in the plate circuit of the last I. F. amplier 2and is also tuned tothe I. F. energy center Y frequency. lThe limiter functions to insure the transmission to the second detector of solely FM signals,no amplitude modulation eiects being transmitted. Those skilled in the art are fully acquainted with the circuit details of the networks prior to the limiter, and hence they need not be described in any further detail.

The limiter stage itself `comprises a tube' 3 which may be a pentode tube, say of the 6S'J7 type. The signal grid 4 may be coupledto the high potential side of the input circuit Iby the condenser 5, the grid vleak resistor 6 connecting the grid end of condenser 5 to cathode. c l

The plate l of the limiter` tube is connected to a source of positive potential throughthe'prithe groundedy mary winding 8 of I. F. transformer 9. The

winding 8 is shunted by a condenser I 0 whichresonates it'to the operating I; F. value. The

screen'grid electrode I I of the limiter tube is con-1 -potentiometer Ri-Rz which is adjustable and a potential which is of a predetermined magnitude.

When theA magnitude of R1 is decreased,y then themagnitudeof R2v is increased. The potentiometer arm A connects the screen lead to Ri-Rz so as to vary the magnitudes of said sections in inverse senses. 'I'he arm A is designated as .Volume control, and it will be understood that adjustment of arm A results in variation of the signal output level at circuit Iii-8. Before describing in'detail the manner in which adjustment of resistor sections R1 and R2 results in variation'of the limiter signal output level, itis rst'pointed out that the network follow.- ing the limiter output circuit; may consist of any desired type of discriminator-'second detector network. For example, the numeral I 4 designates a rectangle within which is shown the typical S-shaped characteristic curve of an FM discriminator-second detector network. Those skilled in the art are fully acquainted with the various types of networks that may be employed between the rst audio frequency amplifier and the limiter output circuit lll-8.

It is sufficient for the purpose of the present application, since the invention is conned to the limiter stage, to point out that the input circuit I5 of discriminator-second detector I4 is tuned to the operating I. F. value, and the entire network may follow the teachings disclosed by S. W. Seeley, in U. S. Patent 2,121,103, patented June 2l, 1938. In this type of circuit there is produced across opposed load resistors of a pair of rectiers rectified voltages in polarity opposition. The substantially S-shaped characteristic curve which relates carrier frequency deviation as abscissae to rectified output voltages as ordinates is shown within the rectangle I5. Oi course, there may be employed in place of the Seeley type of network a pair of rectiers which have input circuits'oppositely and equally mistuned from the center frequency of the FM signals. As stated previously, those skilled inthe art are fully aware of the construction of such discriminator-second detector networks, and they all function to produce in the output circuit the modulation voltage components which correspond to the frequency deviations of the FM' carrier.4 The audio frequency voltage developed in the output circuit of the second detector may be impressed upon one or more stages of audio frequency amplification, and such stages may be followed by any desired type of audio reproducer.

Returning now to the limiter stage, it is first pointed out that the stage provides a certain amount of gain over the range of frequency represented by the I. F. peak and the maximum frequency deviation. The primary function of the tube is to act in the manner of an amplier which is very easily overloaded. That is, grid current is caused to iiow and plate current saturation is produced on one half of the signal cycle, and plate current cut-off is created on the other half of the cycle. The essential purpose of the tube is to wipe out all amplitude variations present in the output of the I. F. amplier network ahead of the limiter, and to pass on to the discriminator a constant amplitude-variable frequency signal. The amplitude variations may be created by noise impulses, fading or by the selectivity characteristics of the resonant circuit preceding the limiter tube. The method of creating thefrequired operating conditions for the limiter tube is by a combination of constants. When using a tube of the 6SJ'7 type, which is a sharp cut-off tube, there is employed comparatively low values of screen and plate voltage, and' no initial control grid bias, so that the tube will overload relatively easily and' plate current cut-oli is quickly reached. VThe operating point is established at the center of the linear portion ofthe plate current-grid voltage curve by the application of the correct operating potentials. Under proper conditions signals` rof a comparatively small amplitude will swing the gridvolt'age into that zone which causes plate current cut-oil and plate current saturation thereby limiting the magnitude of signal plate current variations in the plate circuit. The tube does not function in any way to limit the frequency deviation ofthe modulated I. F. signal.

If there are a number of noise peaks included in the FM signalA the limiting action of the tube will result in clipping oii'those peaks which have the limiting action unimpaired. frequency deviation is assumed constant forl all an amplitude exceeding the saturation point of the tube characteristic. Hence, it is seen that by virtue of the limiting action of the tube any signal or noise voltage in excess of that required to drive the grid of the tube to plate current cut-off in the negative region, and to plate current saturation in the positive region, will not cause corresponding changes in the plate current. Hence, the signal and noise peaks to the left of the cutoi point, and to the right of the saturation point, are effectively wiped out of the output signal, which, though it corresponds in frequency to the input Signal, does not vary in amplitude. The'limiter behaves somewhat in the fashion of an automatic volume control circuit. That is, the output signalof the limiter remains practically constant for any increase in the amplitude in the input signal above that required for limiting action. Therefore, the output of the limiter is substantially independent of input, and the signal voltage fed to the discriminator-second detector` will be of constant amplitude for allv signal voltages above the limiting threshold.

The actual signal level which may occur` in the' limiter output circuit is dependent upon the changes in plate current. This output level' can be raised or lowered by increasing or decreasing the operating voltages of the limiter tube. The effect of increasing the magnitude of the screen voltage would be equivalent to moving apart the points of plate current cut-off' and plate current saturation, and, therefora-increasing the spread between the threshold limits in the negative and positive regions of the plate current-grid voltage characteristic of the tube. This would call for a signal of greater amplitude to produce the limiting action, but it'would also result in a correspondingly larger signal output. On the other hand, if the limiter tube screen voltage were reduced the plate current cut-off and plate current saturation points would be brought closer together. In such a case a signal voltage of much less amplitude would sui'ce to produce the limiting action, but the signal output of the limiter would be correspondingly reduced.

In Fig. 2 there is shown a family of limiter signal input signal output characteristics. It will be realized, of course, that the limiter signal output is equivalent to the output existing atthe output circuit of the second detector. The curvesy show the change which occurs in the limiting action of the tube as therlimiter screen potential is changed from +10 volts to +150 Volts. It will be observed that as the screen potential is raised the signal output level at the limited output circuit is similarly raised, but that the effective threshold point is also increased.

In a limiter of the screen grid type the plate current cut-off value is largely dependent on the screen potential, and, hence the detector outtially independent of input signal. Consequently,

the adjustment of screen potential in the limiter tube maybe used to control the output from the detector while at the same time maintaining In Fig. 2 the curves. Signals below the limiter threshold value are usually so marred by noise impulses as to lack entertainment value. There is some shift in threshold value with screen Voltage adjustment; but most signals which arelistened to are above the'threshold-value foreven the higher voltages. Hence, the-dotted vertical lline in Eig. 2 has been indicated as Effective thresholdf *I It is desired that the detector output, for a given frequency deviation, be affected only -by the'setting of the screen-potential control arm A, and not byvany variation in signal level for signalsabove the threshold value. Changes in signa1` level affect the limiter grid bias, and, hence, thefscreen current. In'order that this variation in screen current be prevented from aiecting the scr'een potential materiallyrthe screen should be fed from a'source" of good'regulatidm For example, such a source,as shown, is a potential divider `Which carriesY current appreciably larger than the maximum screen current. -If the screen potential is too great-then" limiting action would be impaired. The resistor section Rs is ordinarily chosen so that the maximum screen potential obtainable by adjustmentL ofthe volume control does not exceed the value for satisfactory limiting action. This is necessary to prevent the maximum current that may be drawn by the screen from exceeding the safe maximum rating. Some trace of this effect is illustrated in the curve representing +150 volts applied to the screen. For still higher screen voltages, with the circuit parameters assumed, the limiting action Will be still less effective, so it is desirable to limit the maximum screen potential.

A plate dropping resistor 20 of about 100,000 ohms may be inserted in the plate lead of the limiter tube. With zero signal applied to the grid 4 the plate voltage is of the order of 10 volts. The plate voltage rises to some '70 volts as signal is applied, and depends, of course, on the relative magnitude of the signal. This, also, depends upon the potential applied to screen grid I l. Fig. 3 indicates the manner of variation of plate potential as screen potential is varied in the limiter.

While We have indicated and described several systems for carrying our invention into effect, it Will be apparent to one skilled in the art that our invention is by no means limited to the particular organizations shown and described, but that many modifications may be made Without departing from the scope of our invention, as set forth in the appended claims.

What We claim is:

1. In combination with an electron discharge tube having signal input and output circuits, means for applying to said input circuit a signal of variable frequency and variable amplitude, said tube comprising at least a cathode, a signal grid, a screen grid and anoutput electrode,

means for applying signals in said input circuit to said signal grid, means for connecting said output electrode to said output circuit, means forr applying to said output electrode and screen grid positive direct current potentials, said screen potential being so chosen that said tube functions as a limiter thereby to remove from said variable frequency signal amplitude variations above a predetermined threshold value, and a manual volume control means comprising a device for varyingv the magnitude of the screen grid potential over a Wide range of positive values without impairing the limiting action of the tube at selected screen grid potentials.

2. In combination With an electron discharge tube having signal input and output circuits, means for applying to said inupt circuit a signal of variable frequency and variable amplitude, said tube comprising at least a cathode, a signal grid, a screen grid and an output electrode,

means for applying Y signals Q-inl fsaid' input(` circuit tosaid signal grid, means-forconnecting said output electrode 'to' said'output circuit, means for applying to said output electrode 'and screen grid positiveV direct current potentials, said screen potential being "so chosen that said tubefunctions asa limiter'thereby to remove from said variable frequency signal amplitude variations above' a predetermined' threshold value, and av manual volume control device` for `varying vthe magnitude of the screen grid potential -Without impairing the limiting action of the tube at selected screen.y

grid potentials,V said device comprising a poten'- tial i divider connected with "said Sp'otentialapplying! means,saiddivider including-'at least two series resistors, and means forl'varying the magnitudes of said resistors -in opposing senses therebyy tojvarythefscreenfpotentialover av Wide range oflpositive values. 'L' `i y* 3.!Incombination with an electron discharge tube having. 'signal input Aand output circuits, means.' forv applying to :said input circuit'ia signal of variable frequency and variable amplitude,

said tube comprising at least a cathode, a signal grid, a screen grid and an output electrode, means for applying signals in said input circuit to said signal grid, means for connecting said output electrode to said output circuit, means for applying to said output electrode and screen grid positive direct current potentials, said screen potential being so chosen that said tube functions as a limiter thereby to remove from said variable frequency signal amplitude variations above a predetermined threshold value, and a manual volume control device for varying the magnitude of the screen grid potential over a Wide range of positive values Without impairing the limiting action of the tube at selected screen grid potentials, said varying device comprising a potential divider included in said potential applying means, said divider consisting of a resistor section of constant value in series with a resistor of adjustable magnitude, and the screen being connected to the variable resistor.

4. In combination with the' limiter tube of a frequency modulation receiver having signal input and output circuits, means for applying to said input circuit a signal of variable frequency and variable amplitude, said tube comprising at least a cathode, a signal grid, a screen grid and an output electrode, means for connecting said signal grid to said input circuit, means for connecting said output electrode to said output circuit, means applying to said output electrode and said screen grid positive direct current potentials, means for maintaining said signal grid at a relatively small bias with respect to the cathode, said screen potential being so chosen With respect to said grid bias that said tube functions as a limiter thereby to remove amplitude variations above a predetermined thresholdvalue from said signals, and volume control means for the receiver comprising a device for varying the magnitude of the screen grid potential over a Wide range of positive values Without impairing the limiting action of the tube at selected screen grid potentials.

5. In combination with an electron discharge tube having signal input and output circuits, means for applying to said input circuit a signal of variable frequency and variable amplitude, said tube comprising at least a cathode, a signal grid, a screen grid and an output electrode, means for applying signals in said input circuit to said signal grid, means for connecting said output electrode to said output circuit, means for tential beingso chosenthat said tube functionsY a's a limiter'thereby to remove from said variable frequency signal amplitude variationsabove a predetermined threshold value, and a manual gain control device for varying the magnitude of the screen grid potential' over ay Wide 'range of positive values withoutim-pairing the limiting action'of the tube at selected screengrid potential-s and said potentialapplying.means comprising a potentialdivider between said output'electrode and cathode, said dividerA including at least two resistors of adjustable magnitude, andA said control device varying the magnitudes of said resistors in opposing senses-.-

6'.l In combination` Withfan electron discharge tube having. signal input and output circuits, means for applying to said input circuit a signal of' variable frequency and variable' amplitude, saidtube comprising at least a'cathode; a signal grid, afscreen'grid-and' anv output electrode, means for applying signals in said inputcircuit-to said signal grid, meansforconnectingl said output electrode to said output circuit, means for main-v taining said output electrode at a positive direct current potential, means for maintaining said.

screen grid at a positive direct current potential, means for maintaining said signal grid ata bias close to the cathode, and said screen potential being so chosen with respect to said grid bias that said-tube functions as a limiterl thereby to` remove amplitude Variations from said variable frequency signal above apredetermined threshold value, and said screengrid potential applying.

meansfincluding again control means, the latter comprising a device for varying the magnitude of theA screen grid potentialover a wide range ofl positive'values Without impairing the limiting 

